Re: [PATCH v3 bpf-next 15/18] bpf: Support attaching tracing BPF program to other BPF programs

[Song Liu <songliubraving@fb.com>]

> On Nov 7, 2019, at 10:40 PM, Alexei Starovoitov <ast@kernel.org> wrote:
> 
> Allow FENTRY/FEXIT BPF programs to attach to other BPF programs of any type
> including their subprograms. This feature allows snooping on input and output
> packets in XDP, TC programs including their return values. In order to do that
> the verifier needs to track types not only of vmlinux, but types of other BPF
> programs as well. The verifier also needs to translate uapi/linux/bpf.h types
> used by networking programs into kernel internal BTF types used by FENTRY/FEXIT
> BPF programs. In some cases LLVM optimizations can remove arguments from BPF
> subprograms without adjusting BTF info that LLVM backend knows. When BTF info
> disagrees with actual types that the verifiers sees the BPF trampoline has to
> fallback to conservative and treat all arguments as u64. The FENTRY/FEXIT
> program can still attach to such subprograms, but won't be able to recognize
> pointer types like 'struct sk_buff *' into won't be able to pass them to
					^^^^^ these few words are confusing
> bpf_skb_output() for dumping to user space.
> 
> The BPF_PROG_LOAD command is extended with attach_prog_fd field. When it's set
> to zero the attach_btf_id is one vmlinux BTF type ids. When attach_prog_fd
> points to previously loaded BPF program the attach_btf_id is BTF type id of
> main function or one of its subprograms.
> 
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> 

[...]

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index cd9a9395c4b5..f385c4043594 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9390,13 +9390,17 @@ static void print_verification_stats(struct bpf_verifier_env *env)
> static int check_attach_btf_id(struct bpf_verifier_env *env)
> {
> 	struct bpf_prog *prog = env->prog;
> +	struct bpf_prog *tgt_prog = prog->aux->linked_prog;
> 	u32 btf_id = prog->aux->attach_btf_id;
> 	const char prefix[] = "btf_trace_";
> 	struct bpf_trampoline *tr;
> 	const struct btf_type *t;
> +	int ret, subprog = -1, i;
> +	bool conservative = true;
> 	const char *tname;
> +	struct btf *btf;
> 	long addr;
> -	int ret;
> +	u64 key;
> 
> 	if (prog->type != BPF_PROG_TYPE_TRACING)
> 		return 0;
> @@ -9405,19 +9409,42 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> 		verbose(env, "Tracing programs must provide btf_id\n");
> 		return -EINVAL;
> 	}
> -	t = btf_type_by_id(btf_vmlinux, btf_id);
> +	btf = bpf_prog_get_target_btf(prog);

btf could be NULL here, so we need to check it?


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